1. Field of the Invention
The present invention relates to a rotary magnetic recording and reproducing apparatus which may be suitable to be used in a video tape recorder (VTR) or digital audio tape recorder (DAT). In particular, the present invention relates to a structure of a stationary cylinder of a cylinder drum of a rotary magnetic recording and reproducing apparatus, which is effective to relax shock or vibration of a magnetic tape when a magnetic head mounted on a rotary cylinder of the cylinder drum leaves from the magnetic tape and to improve the so-called "head-touch" of the magnetic tape in an area immediately before the magnetic head leaves from the magnetic tape to thereby improve a recording and reproducing performance of the rotary magnetic recording and reproducing apparatus.
2. Prior Art
In a rotary magnetic recording and reproducing apparatus of such type as mentioned, a recording or reproducing operation is performed by making a magnetic head mounted on a rotary cylinder rotating at high speed come in relative slide contact with a magnetic tape running along an outer peripheral surface of the rotary cylinder and along an outer periphery of a stationary cylinder disposed coaxially below the rotary cylinder, with a lower edge of the tape being guided along a helical lead formed on a lower portion of the outer peripheral surface of the stationary cylinder.
It is well known that, in such rotary magnetic recording and reproducing apparatus, the magnetic tape is mechanically vibrated when an upper edge of the magnetic tape which is descending along the outer peripheral surface of the rotary cylinder leaves from the magnetic head mounted on a lower edge portion of the rotary cylinder. Such mechanical vibration of the magnetic tape is due to a difference in pressure exterted in the radial direction between the magnetic tape on the stationary cylinder and a top end portion of the magnetic head which is slightly protruding from the outer peripheral surface of the rotary cylinder.
In order to relax this mechanical vibration of the upper edge portion of the magnetic tape, it is proposed in U.S. Pat. No. 4,833,562 issued to M. Kochi et al. and assigned to the same assignee as of this application that there be added of a small protrusion or bulge on an upper edge portion of an outer peripheral surface of the stationary cylinder to reduce the difference in pressure in the radial direction between the magnetic tape and the top end portion of the magnetic head before the magnetic head leaves from the upper edge portion of the magnetic tape to thereby restrict such mechanical vibration of the magnetic tape.
The conventional technique disclosed in the above mentioned reference in which mechanical vibration of the magnetic tape is restricted by providing the bulge will be described in detail with reference to FIGS. 12(a) to 16.
FIGS. 12(a), 12(b) and 12(c) are an enlarged plan view, an enlarged front view and an enlarged cross sectional view taken along a line c--c in FIG. 12(b), of a tape leaving portion of a conventional stationary cylinder of a rotary magnetic recording and reproducing apparatus shown in the above mentioned reference, respectively. The expression "tape leaving portion" means a portion of the stationary cylinder where a magnetic tape leaves from the stationary cylinder.
In FIGS. 12(a) to 12(c), the stationary cylinder 1 has a helical guide lead B formed in a lower portion of an outer peripheral surface thereof for guiding a lower edge of the magnetic tape and defining a lower edge of a tape running surface area A. In the tape leaving portion of an upper edge portion of the tape running surface area A, a bulge C is formed. The bulge C may be formed by machining the tape running surface area A and the tape lead B of the outer peripheral surface of the stationary cylinder except a corresponding portion thereof to the bulge C. The bulge C functions to push the magnetic tape outwardly in the radial direction in the tape leaving portion out to substantially the same radical height as that of the magnetic head to thereby at least reduce mechanical vibration of the magnetic tape when an upper edge portion of the magnetic tape leaves from the magnetic head on the rotary cylinder 2. That is, as shown by a dotted curve Q in FIG. 13(a) which corresponds to FIG. 10 of the above mentioned U.S. patent which shows the relationship of head surface pressure with respect to tape wind angular position and by a dotted curve S in FIG. 13(b) which corresponds to FIG. 11 of the same U.S. patent which shows the relationship of tape vibration with respect to tape wind angular position, the head surface pressure is reduced by the provision of the bulge formed at angular position of 180 corresponding to the tape leaving position and the tape vibration is at least reduced by the same bulge.
That is, as shown in FIG. 14(a) which is a cross section of an assembly of a stationary cylinder 1 and a rotary cylinder 2 having a magnetic head 6 of a conventional rotary magnetic recording and reproducing apparatus having such structure as shown in FIGS. 12(a) to 12(c), illustrating a contact between a magnetic tape 16 and the magnetic head 6 mounted on the rotary cylinder 2 when the magnetic head 6 is in an upstream side of the tape leaving portion of the stationary cylinder 1 and in slide contact with the magnetic tape, the upper portion of the magnetic tape is bent radially outward by the magnetic head 6. Therefore, if the magnetic tape descends as it is, the upper edge thereof shall be dropped radially inward when the contact between it and the magnetic head is broken. However, as shown in FIG. 14(b) which illustrates a contact between the magnetic tape 16 and the magnetic head 6 in an area in which the magnetic head 6 is about to leave from the magnetic tape 16, the bulge C formed in the head leaving portion of a tape running surface area A of the stationary cylinder 1 raises the upper portion of the magnetic tape 16 up to substantially the same radially outward level as the magnetic head 6 before it leaves from the magnetic tape 16. Therefore, the head surface pressure exerted by the magnetic tape 16 acting on the magnetic head 6 is reduced enough to relax mechanical shock or vibration of the magnetic tape 16 when the latter leaves from the magnetic head 6. Such bulge C is effective when the magnetic tape 16 is relatively flexible.
In such conventional technique, however, when the magnetic tape is a metal evaporation (ME) tape which is prepared by evaporating magnetic material on a relatively thick plastic base plate, an upper edge portion of the stiff ME tape 16 is pushed out by the magnetic head 6 from the surface of the rotary cylinder 2 as shown in FIG. 5(a) to be described later, slightly before the tape reaches the tape leaving position as shown in FIG. 14(a), resulting in degradation of the so called "head-touch".
FIG. 15 shows a portion of the magnetic head 6, including a magnetic or working gap H in which signal is transferred between the head and the magnetic tape, wherein area G', indicated by a chain line, is a slide contact area in which the magnetic tape 16 is in slide contact with the magnetic head 6 when the relative position of the magnetic tape 16 to the magnetic head 6 is as shown in FIG. 14(a). That is, when the tape 16 is stiff, it does not contact with a whole portion of the working gap H of the magnetic head 6 before the latter reaches a position corresponding to the tape leaving portion on the stationary cylinder. Therefore, it is impossible to obtain acceptable recording and reproducing characteristics of the rotary magnetic recording and reproducing apparatus.
FIG. 16 illustrates a recording pattern including magnetically recorded tracks 12a', 13a', 12b' . . . on a magnetic tape running along a conventional tape running surface of the stationary cylinder mentioned above, when viewed from a diamagnetic surface thereof. In FIG. 16, J' depicts a region including the tape leaving portion of the magnetic head 6. Thus, the positional relationship of the magnetic head and magnetic tape as illustrated in FIGS. 14(a) and 15, results in insufficient magnification of the upper portions of tracks 12a', 1ea', and 12b' as illustrated in FIG. 16.
FIG. 17 shows a signal waveform obtained when the tape 16 having such magnetized pattern as shown in FIG. 16 is reproduced. In FIG. 17, the ordinate shows the output level and the abscissa shows time. K' in FIG. 17 depicts an area in which the magnetic head 6 is separated from the tape 16. FIG. 17, indicates that the output level of signal reproduced from the upper portions of the tracks 12a', 13a', 12b', . . . , is lowered. Such state is displayed on a lower portion of a television screen with low S/N ratio.
As mentioned with reference to FIGS. 12(a) to 17, although the conventional recording and reproducing apparatus in which the bulge C is provided on a tape leaving portion of the tape running surface portion of the outer peripheral surface of the stationary cylinder is very effective to restrict vibration of the magnetic tape in the tape leaving position, there is no consideration as to degradation of the "head-touch" problem which occurs, for example, when the tape used is stiff.
Japanese Patent Application Laid-open No. Hei 4-298846 discloses another conventional technique for reducing vibration of tape when it leaves a video head. In this technique, a recess and a bulge are formed on a stationary cylinder surface in upstream and down stream portions of a tape leaving point, respectively. In this technique, the bulge is formed in order to reduce vibration of the tape caused by the recess which is provided to improve a contact with the tape and a magnetic head. The head-touch problem is also not solved by this technique.